Optical communication systems employ optical amplifiers, e.g., to compensate for signal attenuation in optical fibers. One type of amplifier that may be used in a fiber-based optical communication system is an optical parametric amplifier (OPA). As known in the art, an OPA is a device that produces a tunable coherent optical output via nonlinear optical processes, in which, typically, one or two pump-wave photons are converted into two new photons with conservation of photon energy and momentum. The waves corresponding to the two new photons are usually referred to as a signal wave and an idler wave.
In many optical systems, the required photon frequencies differ from the frequencies at which the transmitters emit photons or the receivers detect them efficiently. For this reason, optical frequency conversion (FC) is important. Parametric amplification (PA) in a fiber is based on four-wave mixing (FWM). Not only does FWM produce amplified signals, it also produces idlers that are frequency-shifted and, in some cases, phase-conjugated images of the signals. Consequently, PAs have many uses in classical communication systems. However, with amplification comes noise. Although this noise might not impair the performance of classical (many-photon) systems, it does change the characteristics of quantal (few-photon) systems. Consequently, PAs must be used with caution in few-photon systems.